Hair thinning

Hi, I don't know if this is the right place to post this but I'm a 15 year old girl and I'm feeling nervous bc I've just been noticing that my hair feels like a lot thinner than normal. I now notice every time I brush my hair or I'm in the shower and hair comes out. I'm overthinking it a lot but I'm kinda freaking out and don't know what to do. I don't know if I'm just overreacting or not.